No. 38–Science vs. Assumptions
Stuck for an idea for the topic of this month’s column and with the deadline approaching, I turned to my old friends Ruppert, Fox and Barnes* for inspiration. I’ve used their nearly thousand page tome in the past to research specific creatures but never looked at the book as a whole.
On reading page one for the first time I was somewhat taken aback by the estimate of 30,000,000 for the number of animal species currently living on earth. Of those, 99% are invertebrates and fewer than 1 million of all species have been identified. It gives one pause to think how much work lies ahead for future zoologist if earth’s creatures are to be fully cataloged. Sadly, that work may become less daunting if already alarming extinction rates continue to accelerate.
The remainder of chapter one was an explanation of the scientific methods by which species are categorized. The only things clear after the first reading were a feeling of what am I getting myself into and that in order to gain even a cursory understanding of these concepts that the first challenge for a mind sadly lacking in scientific language skills was to come to grips with terms like phylogeny, cladistics and autapomorphy. Not to mention paraphyletic, homology and synapomorphy. They are brain as well as tongue twisters.
By the fifth reading new neural pathways were beginning to make connections and the veil began to lift on the science of differentiating and classifying species.
To begin with, species are taxonomically grouped using recognizable shared characteristics. The book uses arthropods as an example. They all have exoskeletons, segmented bodies and jointed appendages.
Phylocenetics is the study of the evolutionary relationships among taxonomic groups for the purpose of creating a phylogeny (a phylogenetic tree – graph). Cladistics is the term used for the method by which the tree is constructed. One exercise is to determine a trait shared by two taxa but found in no others. This trait is called a synapomorphy. An example may be found comparing crustaceans and tracheates (insects). Though they share many characteristics with other arthropods to include exoskeletons and jointed appendages, the one trait they share alone is their mandibles (mouth parts). Because of this the two taxa may be combined to form the monophyletic (single ancestor) taxon mandibulata.
In other words, shrimp and grass hoppers evolved from the common ancestor of crustaceans and tracheates which itself in a quirk of nature called an autapomorphy was the first creature to brandish mandibles.
Staying with the arthropods the authors go on to describe one of the challenges faced by phylogenetic scientist with some 29 million species yet to be identified and classified. Here, leaving as much of the not already introduced arcane jargon as possible behind for the rest of this piece, the problem lies in recognizing false monophyletic taxa assumptions.
The example used compares the arthropod taxon (chelicerea) which contains the horseshoe crab with crustaceans and tracheates. Though the three share a common autamorphic ancestor that had evolved jointed appendages, the limb of the phylogenetic tree splits there into chelicerea and mandibulata branches and never again the twain shall meet.
Over time new species will be found, described, and ascribed as a twig on one branch or another on the phylogenic tree. On occasion a classification will be challenged and by the scientific method be either confirmed or correctly refuted.
The point of this exercise, and what I have taken from it, is a reaffirmation of things not always being what they seem. That’s the inner beauty of science. It’s through the challenge of assumptions, even strongly held ones both in science and the world at large, that the ultimate truth is revealed.
*Invertebrate Zoology, Seventh Edition